Berkeley Lab scientists say marriage opens door to development of single device with exceptional range of optical capabilities

Bringing opposing forces together in one place is as challenging as you would imagine it to be, but researchers in the field of optical science have done just that.

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have for the first time created a single device that acts as both a laser and an anti-laser, and they demonstrated these two opposite functions at a frequency within the telecommunications band.

Their findings, reported in a paper published today in the journal Nature Photonics, lay the groundwork for developing a new type of integrated device with the flexibility to operate as a laser, an amplifier, a modulator, and an absorber or detector.

“In a single optical cavity we achieved both coherent light amplification and absorption at the same frequency, a counterintuitive phenomenon because these two states fundamentally contradict each other,” said study principal investigator Xiang Zhang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This is important for high-speed modulation of light pulses in optical communication.”

Reversing the laser

The concept of anti-lasers, or coherent perfect absorber (CPA), emerged in recent years as something that reverses what a laser does. Instead of strongly amplifying a beam of light, an anti-laser can completely absorb incoming coherent light beams.

While lasers are already ubiquitous in modern life, applications for anti-lasers—first demonstrated five years ago by Yale University researchers—are still being explored. Because anti-lasers can pick up weak coherent signals in the midst of a “noisy” incoherent background, it could be used as an extremely sensitive chemical or biological detector.

(From left) Berkeley researchers Xiang Zhang, Zi Jing Wong, Jeongmin Kim and Yuan Wang stand next to the optical setup they designed to demonstrate both lasing and anti-lasing in a single device. (Credit: Marilyn Chung/Berkeley Lab)

A device that can incorporate both capabilities could become a valuable building block for the construction of photonic integrated circuits, the researchers said.

“On-demand control of light from coherent absorption to coherent amplification was never imagined before, and it remains highly sought after in the scientific community,” said study lead author Zi Jing Wong, a postdoctoral researcher in Zhang’s lab. “This device can potentially enable a very large contrast in modulation with no theoretical limits.”

The researchers utilized sophisticated nanofabrication technology to build 824 repeating pairs of gain and loss materials to form the device, which measured 200 micrometers long and 1.5 micrometers wide. A single strand of human hair, by comparison, is about 100 micrometers in diameter.

The gain medium was made out of indium gallium arsenide phosphide, a well-known material used as an amplifier in optical communications. Chromium paired with germanium formed the loss medium. Repeating the pattern created a resonant system in which light bounces back and forth throughout the device to build up the amplification or absorption magnitude.

If one is to send light through such a gain-loss repeating system, an educated guess is that light will experience equal amounts of amplification and absorption, and the light will not change in intensity. However, this is not the case if the system satisfies conditions of parity-time symmetry, which is the key requirement in the device design.

Balance and symmetry

Parity-time symmetry is a concept that evolves from quantum mechanics. In a parity operation, positions are flipped, such as the left hand becoming the right hand, or vice versa.

Now add in the time-reversal operation, which is akin to rewinding a video and observing the action backwards. The time-reversed action of a balloon inflating, for example, would be that same balloon deflating. In optics, the time-reversed counterpart of an amplifying gain medium is an absorbing loss medium.

A system that returns to its original configuration upon performing both parity and time-reversal operations is said to fulfill the condition for parity-time symmetry.

Soon after the discovery of the anti-laser, scientists had predicted that a system exhibiting parity-time symmetry could support both lasers and anti-lasers at the same frequency in the same space. In the device created by Zhang and his group, the magnitude of the gain and loss, the size of the building blocks, and the wavelength of the light moving through combine to create conditions of parity-time symmetry.

When the system is balanced and the gain and loss are equal, there is no net amplification or absorption of the light. But if conditions are perturbed such that the symmetry is broken, coherent amplification and absorption can be observed.

In the experiments, two light beams of equal intensity were directed into opposite ends of the device. The researchers found that by tweaking the phase of one light source, they were able to control whether the light waves spent more time in amplifying or absorbing materials.

Speeding up the phase of one light source results in an interference pattern favoring the gain medium and the emission of amplified coherent light, or a lasing mode. Slowing down the phase of one light source has the opposite effect, resulting in more time spent in the loss medium and the coherent absorption of the beams of light, or an anti-lasing mode.

If the phase of the two wavelengths are equal and they enter the device at the same time, there is neither amplification nor absorption because the light spends equal time in each region.

The researchers targeted a wavelength of about 1,556 nanometers, which is within the band used for optical telecommunications.

“This work is the first demonstration of balanced gain and loss that strictly satisfies conditions of parity-time symmetry, leading to the realization of simultaneous lasing and anti-lasing,” said study co-author Liang Feng, former postdoctoral researcher in Zhang’s Lab, and now an assistant professor of electrical engineering at the University at Buffalo. “The successful attainment of both lasing and anti-lasing within a single integrated device is a significant step towards the ultimate light control limit.”

Tiny units of matter and chemistry that they are, atoms constitute the entire universe. Some rare atoms can store quantum information, an important phenomenon for scientists in their ongoing quest for a quantum Internet.

New research from UC Santa Barbara scientists and their Dutch colleagues exploits a system that has the potential to transfer optical quantum information to a locally stored solid-state quantum format, a requirement of quantum communication. The team’s findings appear in the journal Nature Photonics.

“Our research aims at creating a quantum analog of current fiber optic technology in which light is used to transfer classical information — bits with values zero or one — between computers,” said author Dirk Bouwmeester, a professor in UCSB’s Department of Physics. “The rare earth atoms we’re studying can store the superpositions of zero and one used in quantum computation. In addition, the light by which we communicate with these atoms can also store quantum information.”

Atoms are each composed of a nucleus typically surrounded by inner shells full of electrons and often have a partially filled outer electron shell. The optical and chemical properties of the atoms are mainly determined by the electrons in the outer shell.

Rare earth atoms such as erbium and ytterbium have the opposite composition: a partially filled inner shell surrounded by filled outer shells. This special configuration is what enables these atoms to store quantum information.

However, the unique composition of rare earth atoms leads to electronic transitions so well shielded from the surrounding atoms that optical interactions are extremely weak. Even when implanted in a host material, these atoms maintain those shielded transitions, which in principle can be addressed optically in order to store and retrieve quantum information.

Bouwmeester collaborated with John Bowers, a professor in UCSB’s Department of Electrical and Computer Engineering, and investigators at Leiden University in the Netherlands to strengthen these weak interactions by implanting ytterbium into ultra-high-quality optical storage rings on a silicon chip.

“The presence of the high-quality optical ring resonator — even if no light is injected — changes the fundamental optical properties of the embedded atoms, which leads to an order of magnitude increase in optical interaction strength with the ytterbium,” Bouwmeester said. “This increase, known as the Purcell effect, has an intricate dependence on the geometry of the optical light confinement.”

The team’s findings indicate that new samples currently under development at UCSB can enable optical communication to a single ytterbium atom inside optical circuits on a silicon chip, a phenomenon of significant interest for quantum information storage. The experiments also explore the way in which the Purcell effect enhances optical interaction with an ensemble of a few hundred rare earth atoms. The grouping itself has interesting collective properties that can also be explored for the storage of quantum information.

Key is an effect called a photon echo, the result of two distinct light pulses, the first of which causes atoms in ytterbium to become partially excited.

“The first light pulse creates a set of atoms we ‘talk’ to in a specific state and we call that state ‘in phase’ because all the atoms are created at the same time by this optical pulse,” Bouwmeester explained. “However, the individual atoms have slightly different frequencies because of residual coupling to neighboring atoms, which affects their time evolution and causes decoherence in the system.” Decoherence is the inability to keep track of how the system evolves in all its details.

“The trick is that the second light pulse changes the state of the system so that it evolves backwards, causing the atoms to return to the initial phase,” he continued. “This makes everything coherent and causes the atoms to collectively emit the light they absorbed from the first pulse.”

The strength of the photon echo contains important information about the fundamental properties of the ytterbium in the host material. “By analyzing the strength of these photon echoes, we are learning about the fundamental interactions of ytterbium with its surroundings,” Bouwmeester said. “Now we’re working on strengthening the Purcell effect by making the storage rings we use smaller and smaller.”

According to Bouwmeester, quantum computation needs to be compatible with optical communication for information to be shared and transmitted. “Our ultimate goal is to be able to communicate to a single ytterbium atom; then we can start transferring the quantum state of a single photon to a single ytterbium atom,” he added. “Coupling the quantum state of a photon to a quantum solid state is essential for the existence of a quantum Internet.”

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